A monoclonal antibody (anti 3MC 1-7-1) raised against a cytochrome P-450 induced by 3-methylcholanthrene in rats, blocked the conversion of R and S propranolol to 4-hydroxypropranolol (40HPr), 5-hydroxypropranolol (50HPr), and desisopropylpropranolol (DIP) by microsomes from the small intestine of rats treated with Beta-naphthoflavone (BNF). The ratios of the products formed from R-propranolol, namely 40HPr/50HPr and DIP/50HPr, by the antibody sensitive portions of the enzymatic activities in the microsomes of liver, kidney, lung and small intestine of BNF-treated rats were remarkably similar. In addition, this monoclonal antibody blocked effectively the conversion of R-Pr to 40HPr, 50HPr and DIP in microsomes from stomach and colon of rats.